The present invention relates to a device for detecting the temperature of an electromagnetic clutch for vehicular use. More particularly, it relates to a temperature detecting device which can precisely detect the temperature of an electromagnetic clutch mounted on a vehicle, by calculating a change in the electric resistance of the clutch based on the clutch current and the clutch voltage.
An example of a conventional clutch controller, as illustrated in FIG. 3, which can be utilized for detecting the clutch temperature, is described in Japanese Patent Laid-Open No. 63-57342. In FIG. 3, the clutch controller illustrated includes a clutch current control means 1, a clutch current calculating means 2 such as a microcomputer, a power source 3 in the form of a battery mounted on a vehicle, and a vehicular electromagnetic clutch 4.
The clutch current calculating means 2 receives a travel control information signal SD and an engine control information signal SE, and generates, based on these signals SD and SE, a clutch current command signal SI which is inputted to a positive input terminal of a deviation amplifier 14 of the clutch current control means 1, and a clutch cut-off signal SO which is supplied to the base of an NPN transistor 12, which functions as an output transistor, inside the clutch current control means 1. The NPN transistor 12 has the emitter thereof connected to the negative terminal of the battery 3 and to ground through a clutch current sensing resistor 13. The emitter of the NPN transistor 12 is also coupled with the negative input terminal of the deviation amplifier 14 so as to input a current feedback signal SF. The deviation amplifier 14 generates a deviation between the clutch current command signal SI and the current feedback signal SF and outputs a signal indicative of the result of calculation via a pulse width modulator (PWM) 15 to the base of a PNP transistor 11, which functions as an output transistor, for turning it on and off. The PNP transistor 11 has the emitter connected with the positive terminal of the battery 3, and the collector connected to ground via a parallel circuit comprising a circulation diode 16 and a deexcitation resistor 18, and at the same time to an output terminal 20 of the clutch current control means 1.
The NPN transistor 12 has the collector connected with the positive terminal of the battery 3 through a parallel circuit comprising a deexcitation resistor 19 and an overcurrent suppression diode 17, and at the same time with an output terminal 21 of the clutch current control means 1.
The deexcitation resistors 18, 19 function to supply a slight amount of deexcitation current to the electromagnetic clutch 4 when both the PNP and NPN transistors 11, 12 are off.
The output terminals 20, 21 of the clutch current control means 1 are connected with power supply elements 42a, 42b for the electromagnetic clutch 4. The clutch 4 comprises an excitation coil 41 and the power supply elements 42a, 42b connected in series thereto.
In operation, the clutch current calculating means 2 calculates the speed of the vehicle, the number of revolutions per minute of the engine, and a clutch torque based on the travel control information signal SD and the engine control information signal SE. The clutch current calculating means 2 then generates an output signal of a wave form (i.e., in the form of a clutch current) corresponding to the clutch torque thus calculated.
When the electromagnetic clutch 4 is cut off or disengaged, the PNP transistor 11 and the NPN transistor 12 are turned off by the output signal of the pulse width modulator 15 and the clutch cut-off signal SO from the clutch current calculating means 2, respectively, so that a slight or limited deexcitation current is caused to flow through the electromagnetic clutch 4 via the deexcitation resistor 18, 19 in the reverse direction. On the other hand, when the electromagnetic clutch 4 is engaged, the NPN transistor 12 remains on so that the clutch current flowing through the electromagnetic clutch 4 is sensed by the clutch current sensing resistor 13 as a clutch voltage. The clutch voltage across the clutch current sensing resistor 13 is imposed on the negative input terminal of the deviation amplifier 14, as a current feedback signal SF, which receives, at its positive input terminal, the clutch current command signal SI from the clutch current calculating means 2. The deviation amplifier 14 compares the clutch current command signal SI and the current feedback signal SF and generates an output signal indicative of a deviation therebetween, which is outputted to the pulse width modulator 15. The pulse width modulator 15 modulates the pulse width of the output of the deviation amplifier 14 and sends an output signal to the base of the PNP transistor 11 whereby the transistor 11 is turned on and off depending on the pulse width of the pulse width modulator output, so as to control the current supply to the electromagnetic clutch 4. When the PNP transistor 11 is off, a circulating current flows through the circulation diode 16.
FIG. 4 shows another example of a conventional clutch controller having a clutch temperature detecting function which is disclosed in Japanese Patent Laid-Open No. 60-98822. The clutch controller illustrated in FIG. 4 is substantially similar to that of FIG. 3 except for a clutch voltage sensing circuit. Thus, the same or corresponding elements are identified by the same reference numerals. The clutch controller of FIG. 4 includes a deviation amplifier 14 and a pulse width modulator 15 which are more concrete in arrangement than, but substantially similar in operation to, those of the previously described example of FIG. 3. Specifically, in FIG. 4, a clutch current command signal SI generated by a clutch current calculating means (not shown) is inputted to the positive input terminal of the deviation amplifier 14 via a resistor 14a. The positive input terminal of the deviation amplifier 14 is connected to the positive terminal of a battery 3 through a pair of serial resistors 14b, 14c. The deviation amplifier 14 has the output terminal coupled via a resistor 15a with the base of a transistor 15b, which is a major component of the pulse width modulator 15, and at the same time with a junction between the resistors 14b, 14c. The transistor 15b has the emitter grounded and the collector coupled with the base of a PNP transistor 11 through a resistor 15c. The base of the PNP transistor 11 is coupled with the emitter thereof via a resistor 11a. The transistor 15b and the resistors 15a, 15c together constitute the pulse width modulator 15.
The PNP and NPN transistors 11, 12 and the circuit around the electromagnetic clutch 4 are illustrated in a much simpler form than those of FIG. 3, but they have substantially the same arrangement and operation. The NPN transistor 12 receives at its base a clutch cut-off signal SO through a resistor 12a.
A voltage sensing circuit 50 comprises a series circuit including a filter 24 and a Zener diode 23 connected between the collector of the PNP transistor 11 and ground. The filter 24 comprises a resistor 24a and a capacitor 24b connected in series with each other with a junction therebetween being connected with the positive input terminal of the comparator 50a which receives, at its negative input terminal, a clutch current command signal SI from the clutch current calculating means 2. The comparator 50a has an output terminal connected with the positive terminal of the battery 3 through a resistor 50b, and at the same time with an output terminal 50c of the voltage sensing circuit 50 for outputting a clutch voltage signal SC2.
Now, explanation will be made of how to detect the temperature of the electromagnetic clutch 4 while referring to FIG. 4. Of course, detection of the clutch temperature is carried out when the electromagnetic clutch 4 is in an engaged state. In this state, current is supplied from the battery 3 to the electromagnetic clutch 4 when the PNP transistor 11 is turned on. The PNP transistor 11 is held on as long as the pulse width of an output pulse of the pulse width modulator 15. As a result, a voltage drop across the electromagnetic clutch 4 is developed due to the pulse-width-modulated exciting current flowing through the excitation coil 41 and it is imposed upon the filter 24. The filter 24 detects an estimated voltage across the excitation coil 41 from the voltage drop across the clutch 4.
Since the voltage drops across the power supply elements 42a, 42b are substantially constant irrespective of the temperature of the electromagnetic clutch 4 and the magnitude of current therethrough, the clutch voltage inputted to the positive input terminal of the comparator 50a is directly affected by the voltage drops across the NPN transistor 12 and the clutch current sensing resistor 13 in the forward direction. Therefore, the forward direction voltage drop across the constant voltage diode 23 is set to be offset by the voltage drops across the power supply elements 42a, 42b, the voltage drop across the NPN transistor 12 and the voltage drop across the clutch current sensing resistor 13 so that an error voltage or a voltage difference between the estimated voltage and the desired average voltage across the exciting coil 41, which is the sum of the voltages across the power supply elements 42a, 42b, the voltage across the NPN transistor 12 and the voltage across the resistor 13, can be eliminated. That is to say, the comparator 50a makes a comparison between the actual voltage drop across the electromagnetic clutch 4, which is imposed on the positive input terminal thereof through the filter 24, and the voltage of the clutch current command signal SI, which is fed from the clutch current calculating means 2, and generates an output signal SC2 when the actual clutch voltage is greater than that of the clutch current command signal SI. Thus, temperature detection of the electromagnetic clutch 4 is effected by detecting a change (i.e., an increase or decrease) in the electric resistance of the clutch 4, which has a certain relationship with the clutch temperature.
With the conventional clutch controller as constructed above, however, the voltage drop between the collector of the PNP transistor 11 and the negative terminal of the battery 3 is fed to the filter 24 where the total sum of the voltage drops across the power supply elements 42a, 42b and the voltage drops across the transistor 12 and the clutch current sensing resistor 13 is offset by the voltage drop across the constant voltage diode 23 so as to provide the clutch voltage across the exciting coil 41. However, since the voltage drops across the NPN transistor 12 and the resistor 13 vary depending on the temperature thereof and the magnitude of current therethrough, it is difficult to control the voltage drop across the diode 23 in such a manner as to exactly match the above total sum of the voltage drops, resulting in poor temperature detection. Thus, if the clutch temperature detected is incorrect, i.e., if the detected clutch temperature is above a prescribed level though the actual clutch temperature is not so high, the current supply to the electromagnetic clutch 4 is interrupted to needlessly disconnect the clutch 4. On the other hand, if the detected clutch temperature is incorrectly below a prescribed level, the electromagnetic clutch 4 may be continuously held connected, resulting in possible overheating.